Rich and poor Americans are slowly but surely staking out separate lives. Increasingly, they have been moving to different communities, and more and more they are also marrying people of similar income and educational backgrounds. This is a phenomenon social scientists call assortative mating.

In 2005, 58 percent of wives with a high school diploma were married to men with the same amount of education, new research by economist Jeremy Greenwood of the University of Pennsylvania and three colleagues shows. In 1960, by contrast, only 42 percent of wives with high school diplomas were married to men with the same level of education.

The phenomenon is happening at the top of the education distribution, too. In 2005, 43 percent of wives with college degrees were married to men who also had college degrees. In 1960, the share was 33 percent.